The invention relates to an inverter which comprises a direct voltage intermediate circuit consisting of a positive and a negative voltage busbar, and an auxiliary voltage source which generates auxiliary voltages referenced to the positive and the negative voltage busbar of the direct voltage intermediate circuit, the inverter also comprising for each phase an upper and a lower power semiconductor, which are connected in series between the positive and the negative voltage busbar, the point between the semiconductors forming the phase output of the inverter; an upper and a lower gate driver the outputs of which are connected to the control electrode of the respective upper and lower power semiconductors and which comprise a positive and a negative auxiliary voltage input and a zero potential point, which is connected to the output electrode of the power semiconductor to be controlled; a first diode the anode of which is connected to the positive auxiliary voltage referenced to the negative voltage busbar and the cathode the positive auxiliary voltage input of the upper gate driver; and a first capacitor which is connected between the positive auxiliary voltage input of the upper gate driver and the zero potential point.
The main circuit of inverters typically consists of a direct voltage intermediate circuit and semiconductor switches connected to it, which are usually FET or IGBT switches. The semiconductor pair connected in series between the positive and the negative voltage busbar of the intermediate circuit forms the phase output of the inverter. The number of such pairs in the inverter depends on the number of output phases, i.e. there is a separate output for each output phase. The semiconductor switches can be used for connecting the voltage of either the positive or the negative voltage busbar to the load connected to the output of the inverter. By connecting voltage pulses to the output the load can be provided with the voltage required at a given time, the amplitude and frequency of which are changeable.
The semiconductors of inverters are controlled by means of gate drivers connected to the control electrode of the semiconductor switches. Depending on the component, the control electrode may be called a gate or a base, for example. The gate drivers generate the necessary current or voltage pulses for the control electrode of the switch, and thus the switch can change over to the conducting state reliably. It is difficult to generate the auxiliary voltage needed by the gate drivers cost-effectively since particularly the potential of the output electrode of the power semiconductor (referred to as an upper semiconductor component in the text) connected to the positive voltage busbar varies considerably as the state of the other power semiconductor changes. The output electrode is called e.g. an emitter or a source, the name varying according to the type of component. Positive turn-on voltage of the upper semiconductor component can be generated using a method known as a bootstrap method, in which positive turn-on voltage is produced from the positive voltage referenced to the negative voltage busbar, in other words, from voltage of a certain magnitude with respect to the negative voltage busbar of the intermediate circuit. In the bootstrap method the upper gate driver receives positive auxiliary voltage through the diode connected to the gate driver when the power semiconductor connected to the negative voltage busbar is in the conducting state. This way it is possible to generate the positive auxiliary voltage which is required by the gate driver and used for igniting the switching component. In low-current power semiconductors the switching component has typically been turned off by connecting the gate to the emitter potential of the component through the base resistance.
If the power semiconductor is intended for high currents, the gate driver must be able to generate a turn-off voltage which is negative with respect to the emitter of the semiconductor. When negative turn-off voltage is used, the component can be made to cut off the passing current quicker. Furthermore, by keeping the gate in the negative potential with respect to the emitter it is possible to prevent unintentional turn-on of the component, which could otherwise be caused by high change rates of the voltage over the component. Changes of the voltage over the component result from state changes of other power semiconductors. In the case of the IGBT, turn-on caused by voltage changes results from rapid change of the internal gate charge due to the influence of the Miller capacitance. Such unintentional turn-on lasts only for a few dozens of nanoseconds during which a high current peak passes through the component, which leads to increased power loss in the power switch. To prevent this phenomenon, the gate of the component must be kept in a negative potential of at least about 5 volts with respect to the emitter when the component is off.
In high-current power semiconductors the negative auxiliary voltage of the upper gate driver is produced for the gate driver using separate transformers or transformer coils for each output phase of the inverter. Such transformers should also withstand the high change rate of the voltage between their primary and secondary coils.